- Infections caused by viruses are common causes of illnesses: the common cold, many ear infections, sore throats, chicken pox, and the flu are caused by different viruses. Usually, these illnesses last only few days or, at most, a few weeks. Some virus infections like influenza are cleared from the body, and others such as the chicken pox virus remain in the body in an inactive state. However, some people may become quite ill when they are infected with a particular virus, possibly because part of their immune system does not respond properly to fight the virus.